Apparatus for diverting and uniformly distributing a vertically downwardly arriving gas stream is required at the entrance cross-section of a horizontal-flow dust-collecting electrostatic precipitator and for effecting a preliminary separation of dust. In such a distributor a flaring conical entrance fitting opens into devices which extend throughout the entrance cross-section of the dust-collecting unit and serve to divert and distribute the gas stream. A housing wall together with the diverting device encloses a downwardly tapering, wedge-shaped entrance passage A dust-accumulating bin is disposed at the bottom of the apparatus.
A distributor is needed when of the direction of flow of the gas stream must be changed from the vertical to the horizontal or when a flow in this latter direction is desirable because it reduces plant costs and pressure drops.
In such cases the diverting and distributing apparatus is generally used as a mechanical preliminary dust separator (see German Utility Model No. 18 64 332 and German Patent Specification No. 10 05 044), particularly if the dust content exceeds 100 grams per cubic meter. Such high dust contents are present when the dust itself is the product of a process, e.g. in the production of cement and the calcination of alumina, rather than dust produced merely as an undesired by-product, e.g., in the operation of power plant boilers or steelmaking converters.
When a gas having a high dust content arrives from above, particularly great difficulties are involved in the deflection of the gas stream through 90.degree. into a horizontal flow and in the uniform distribution of the gas stream over the entrance cross-section of a horizontal-flow dust-collecting unit.
Centrifugal forces and gravitation act in the same sense on the dust particles and tend to segregate them and cause them to become enriched in that portion of the gas stream which flows through the lower portion of the dust-collecting unit.
Also in other applications and for other reasons, the distribution of dust over the flow area of the dust-collecting unit has a strong detrimental effect on the dust-removal capacity because the operation of the dust-collecting unit cannot even approximately be optimized. The much larger quantity of dust collected on the lower portions of the collecting electrodes must be taken into account in the selection of the frequency of the rapping blows performed to clean the electrodes and imposes also an upper limit for the operating voltage.
On the other hand, much less dust is collected on the upper portion of the collecting electrodes and for this reason a different rapping frequency and operating voltage are required for such upper portion. The more divergent the gas flows, the greater is the variation of the quantities of dust collected on the collecting electrodes over the height thereof.
Particularly, the quantities of dust which are released the rapping blows cannot be maximized because this would require an optimum rapping frequency and such optimum rapping frequency cannot be used unless dust is collected with reasonable uniformity over the entire area of the collecting electrodes. If the collection of dust varies greatly over the height of the collecting electrodes, a rapping frequency corresponding to the average dust-collecting rate will be too high in the upper portion and too low in the lower portion.
Additional problems arising in this kind of apparatus are due to an uncontrolled deposition of dust (see German Patent Specification No. 10 05 044). Such deposits of dust may adversely affect the flow of the gas stream and give rise to a need for a removal of the dust. It will be understood that these problems will also be more serious as the dust content of the gas stream increases.